Many millions of tonnes of wood pellets will soon be shipped annually to help keep the lights on in the UK. Other EU nations may follow.

Critics say subsidising wood burning wastes money, does nothing to tackle climate change in the short term, and is wrecking some of the finest forests in the US.

I have tracked the controversial trade from the swamp forests of North Carolina to the towering chimneys of the UK's biggest power station, Drax in Yorkshire, which is converting half its boilers from coal to wood.

The implications are complicated and disputed, but it is clear that EU leaders did not have burning American wood in mind when they mandated that 20% of Europe's energy should come from "renewable" sources.

Environmentalists are preoccupied with the potential effects of the trade on climate change and wildlife.

So I travelled with Drax representatives to a timber operation in southern Georgia run by their main supplier, the timber firm Plum Creek.

It is the biggest private landowner in the US. Its operations are impressive, from the large-scale nurseries selectively breeding seedlings for vigour and disease-resistance, through to immensely productive plantations.

This is among the most effective tree farming in the world. Competition from weeds is eliminated in dark monocrop tree stands where wildlife is scanty.

The plantations are thinned and harvested by mechanical giants which cut and throw whole trees as if they were twigs.

The trees are planted close to each other to encourage tall straight specimens to reach for the light. These will have most value for planks. As the plantation grows, some trees are removed to make space.

The thinned-out trees are of low value. They are traditionally sold to the pulp and paper industry but now there's a new market - power stations in the UK.

The industry contends there is plenty of low-grade material to source pulp and power, but some studies suggest that this may be wishful thinking.

Critics fear that increasing demand from power generators will encourage foresters to take land that is currently growing food.

Their other fear is that plantation forests will replace even more of the natural forests of the southern US, which are already dwindling fast.

I drove with environmentalists at dawn to a gorgeous swamp forest in North Carolina. The birdsong was entrancing, and a scarce prothonatory warbler - known as the swamp canary - danced before our TV lens.

The wood fuel industry has not advertised that it also takes trees from natural forests like this to boil kettles in Britain - but that's what happens. Most of the swamp forests in south-east US are in the hands of small private landowners and they face few restrictions on what they do with their assets.

It is said that local landowners cut their forest twice; once when their daughter gets married and once when they retire. Cutting typically means clear-cutting, and that leaves some left-over, low-value trees for pulp or power.

So is the environmentalist argument really against the power industry? Or against America's laws on forest biodiversity?

It depends, of course, on how much the power industry expands. But the best way of protecting the forests may be for benefactors to buy them, because those wedding dresses will still need to be paid for.

The other big environmental issue is climate change. When the EU set its 2020 target of sourcing 20% of energy from renewables, some leaders thought the deal referred to electricity. (I know because I spoke to Downing Street on the day of the decision).

In fact, it included energy for transport and heating too, so the bar was set much higher than anticipated. Policies create opportunities and entrepreneurs were quick to exploit the potential of wood power, which will soon create more renewable energy in the UK than wind and solar combined.

But will this achieve the stated goal of cutting carbon emissions?

The British government will shortly announce its rules for the sustainability of "biomass" burning for power. It will set a standard for emissions created from the cutting, drying and shipping and timber but it will make a working assumption that burning the wood has nil CO2 emissions as new trees will suck up the CO2 emitted by wood burning.

Critics say this is simplistic as it fails to recognise that it will take maybe 50 years for new trees to absorb the CO2, whilst politicians agree that emissions need to be cut immediately to prevent carbon over-heating the planet.

It also fails to account for the fact that in the US the forest stock has been increasing and this process offsets the growth in carbon emissions from homes and industry. Burning American trees in the UK reduces America's "carbon sink".

Foresters argue that this doesn't matter much as long as the total biomass sent for export is no greater than the wood used in a single large pulp mill. But these numbers will grow fast.

Liberal Democrat MP Nick Harvey has tabled an amendment to the UK Energy Bill insisting that long-term subsidies for biomass burning should only be agreed for plants that capture and store CO2 emissions, or use the waste heat for other purposes. (Drax alone expects subsidies of more than £1bn in coming years from people's electricity bills.)

But the amendment is not supported by any of the major parties.

It looks as though UK Government policy is being driven by the need to hit mandatory targets and keep electrons flowing, rather than by a deep desire to cut CO2 emissions right now.